Electric-Magnetic (EM) inductive handwriting input technique has become widespread in practice. An EM inductive handwriting input device typically includes an EM handwriting pen and an EM position detection input device. Further, based on the relationship between the handwriting pen and the position detection input device, handwriting input devices can be divided into wired EM handwriting input devices, wireless active handwriting input devices and wireless passive handwriting input devices, among which the wireless passive handwriting input devices are most convenient and popular. A wireless passive handwriting pen includes a resonant circuit consisting of an inductor and a capacitor and capable of oscillating in response to sensing a signal transmitted from a position detection input device. Then, a receiver circuit of the position detection device receives an oscillation signal generated by the resonant circuit, such that the position of the handwriting pen relative to the position detection input device can be obtained.
Further, wireless passive handwriting input devices can be sub-divided into analog handwriting input devices and digital handwriting input devices. Compared with analog handwriting input devices, digital handwriting input devices are advantageous in their capabilities of accurately detecting and transmitting pressure information and of detecting and transmitting more key-pressing information for handwriting pens. Digital handwriting pens are provided with CMOS logic circuits and some digital handwriting pens are further provided with processors. FIG. 1 shows a circuit block diagram of an existing digital handwriting pen. In this digital handwriting pen, an oscillation signal generated by a resonant circuit is rectified and filtered and the resulting power source is used to supply power to the circuit of the digital handwriting pen.
In order to improve the detection accuracy of input position of a handwriting pen, a signal amplitude of the handwriting pen needs to be increased, i.e., a voltage amplitude of the oscillation signal generated by the resonant circuit needs to be increased. Currently, the energy of the signal generated by the resonant circuit in the handwriting pen is typically increased by increasing the energy of the signal transmitted by the position detection input device, thereby increasing the voltage amplitude of the oscillation signal generated by the resonant circuit. However, in a digital handwriting input device, a digital handwriting pen uses a large number of CMOS circuits whose current consumption has a non-linear relationship with a power source voltage. Hence, a curve representing the relationship between the energy value of the signal transmitted by the position detection input device and the power source voltage value of the handwriting pen circuit can be obtained, as shown in FIG. 2. It can be seen from FIG. 2 that, after the energy of the signal transmitted by the position detection input device exceeds a point A in FIG. 2, while the energy of the signal transmitted by the position detection input device continues to increase significantly, the power source only increases by a very limited amount and thus the voltage amplitude of the oscillation signal generated by the resonant circuit also increases by a very limited amount. The power source voltage at the point A is herein referred to as an operation voltage of the power source.
Therefore, after the energy of the signal transmitted by the position detection input device has reached a certain level, even if the energy of the signal transmitted by the position detection input device continues to increase significantly, the voltage amplitude of the oscillation signal generated by the resonant circuit substantially remains in a proximity of the operation voltage of the power source. While it is possible to continue to improve the position detection accuracy by increasing the operation voltage of the CMOS circuit, the sensitivity of the digital handwriting pen decreases as the operation voltage of the CMOS circuit increases. Moreover, the operation voltage of the CMOS circuit cannot be increased sufficiently due to limitation in existing processes for the CMOS circuit.